Thermoplastic resins are used in many applications such as household, building industry, automotives and electricity, which require a high degree of flame retardation. According to one of the most common standards used in the industry, namely Underwriters Laboratory (UL) 94 V0, the plastics are required not to develop, during combustion, dripping, which may contribute to the spreading of fire to other materials.
The use of fluoropolymers, particularly the addition of poly (tetrafluoroethylene) (PTFE) fine powder to a flame retardant system is an effective way to prevent the flame dripping of many thermoplastic materials.
It is known in the art to prepare homogeneous mixtures of synthetic polymeric resins with a wide variety of solid phase additives. However, the addition of PTFE fine powder to thermoplastic resins meets with difficulties since the PTFE powder has poor flowability and tends to fibrillate when subjected to even low shear forces. The fibrillated powder blocks the flow of the powder in the mixer or the extruder's feeder and results in low quality compounds with poor dispersion of the PTFE, poor appearance and reduced contribution to flame retardancy. These drawbacks are known and the art has attempted to overcome them.
Representative of prior art processes is that described in U.S. Pat. No. 4,649,168. Accordingly to this patent particles of PTFE are dispersed in aromatic polycarbonate resin-based molding compositions. The dispersion is carried out, in brief, by admixture of aqueous emulsions of the two components followed by coagulation of the emulsion mixture. Coagulation may be carried out by spray-drying, freeze-drying or the addition of inorganic or organic salts, acids, bases or organic solvents which are miscible with water. The process results in fine dispersions of the PTFE in the polycarbonate resin, but the degree of dispersion is dependent upon a number of variables, which require close control.
Other methods to improve the dispersion of PTFE in thermoplastic resins are known in the art.
U.S. Pat. No. 4,753,994 discloses a process for incorporating a fluoropolymer in polycarbonate compositions, comprising adding an aqueous dispersion of a fluoropolymer to a polycarbonate solution, agitating the mixture thus formed, adding a precipitation agent to cause co-precipitation of the polycarbonate and said fluoropolymer, filtering the co-precipitate, washing and drying the filtrate.
U.S. Pat. Nos. 5,521,230 and 6,469,072 disclose a method for dispersing solid forms of additives in polymers, which involves adding dispersions or solutions of additive(s) to a solution of polymer in a tubular mixer (preferably in the presence of a stationary mixer). The mixer leads to a steam precipitation step, wherein all fluid ingredients in the mixture are volatilized leaving the solid additive and resin in the desired ratio.
Molded parts prepared from the compositions of the U.S. Pat. No. 4,753,994 are characterized by an improved homogeneity and flame retardancy. The process described in U.S. Pat. Nos. 5,521,230 and 6,469,072 preserves the physical properties of thermoplastic polymer matrix to which the additives described therein have been added, thanks to the uniform dispersion of said additives in said polymer matrix. However, all the above processes involve the use of organic solvents that need to be removed from the final product by evaporation, and therefore are expensive and highly specialized.
U.S. Pat. No. 4,579,906 discloses a process for making ABS molding materials by mixing an aqueous dispersion of PTFE powder, stabilized with an ethoxylated nonyl phenol surfactant, with a latex of one or more graft polymers (“graft rubbers”) or a latex of one or more matrix resins (“SAN”) or a latex mixture of both, and coagulating the polymer mixture, or optionally mixing said mixture with further graft polymer and/or matrix resin and with inorganic synergists for improving flameproofing and organic halogen compounds.
U.S. Pat. No. 6,040,370 relates to the use of fluoropolymers as additives in thermoplastic resin compositions and, more particularly, a stabilized aqueous fluoropolymer dispersion which includes a fluoropolymer and a fatty acid salt, and to a method for making a thermoplastic resin composition comprising a fluoropolymer additive. The fluoropolymer additive is made by combining an aqueous fluoropolymer dispersion with a second polymer, particularly a styrene-acrylonitrile resin, and precipitating and drying the resulting combination.
EP 0550204 describes polyphenylene ether flame-retarded resins which have been rendered non-dripping through the inclusion of a drip-inhibiting amount of high molecular weight polyethylene resin. It discusses prior art that achieves non-dripping through the use of polytetrafluoroethylene, which however is said to be extremely costly and difficult to blend with polyphenylene ether resins.
EP 0861856 describes the preparation of PTFE powder that is more flowable. However, the preparation is made in a highly specialized equipment and the flow of the powder is not as good as desired.
However, prior art methods present several drawbacks. The use of ethoxylated nonyl phenol surfactants is under scrutiny from the perspective of environmental safety, and alternative approaches to stabilizing aqueous fluoropolymer dispersions are highly desirable. Co-coagulated fluoropolymer-thermoplastic resin compositions tend to be very difficult to handle due to clumping and poor flowability and it is correspondingly difficult to incorporate such additives uniformly and reproducibly into a thermoplastic resin composition. Non-uniform distribution of fluoropolymer additive within a thermoplastic resin composition may result in, e.g., surface imperfections, such as e.g., streaking and splay, and in inconsistent combustion performance, such as, e.g., uneven shrink rates and dripping.
In industrial manufacturing, economy, productivity and working efficiency must be taken into consideration. Even though the prior art improves some aspects related to the dispersion of conventional polytetrafluoroethylene in mixtures with synthetic, thermoplastic, polymer, it does not overcome the aforementioned drawbacks, leading to less economical and productive processes than desired. Moreover, PTFE concentrates in powder form, produced according to the prior art, tend to segregate from dry blended mixtures of flame retardants and/or resins and/or plastic additives.
A fluoropolymer-thermoplastic resin additive, that is in the form of a free-flowing powder, would be highly desirable from both the perspective of material handling and improving the uniformity and reproducibility of the thermoplastic resin compositions made therefrom.
It is therefore an object of this invention to provide an economic and efficient process for introducing a fluoropolymer into a flame retarded thermoplastic resin and evenly disperse it throughout the polymer.
It is another object of this invention to provide a fluoropolymer concentrate that is easily compounded with a base thermoplastic resin.
It is a further object of this invention to provide a thermoplastic resin composition that is flame retarded and includes anti-dripping agent-that is free from the defects of known such compositions, in particular is free from surface imperfections and inconsistent combustion performance.
It is a still further object of this invention to provide a process for improving the PTFE fine powders flowability when they are dispersed as additive into thermoplastic resins.
It is a still further object of the present invention to provide an antidripping agent having excellent handling characteristics and dispersibility while maintaining antidripping property, as well as flame retardant resin compositions containing the antidrippping agent.
Other objects and advantages of the invention will become apparent as the description proceeds.